Uniform heating is important to many industrial applications, including chemical vapor deposition processes as applied to thin substrates such as Si or GaN based wafers. In some chemical vapor deposition systems, one or more wafer substrates are typically placed in a reactor containing a wafer carrier having one or more wafer compartments or wafer pockets. The wafer carrier and wafer substrates are then heated in the reactor, and are subjected to gasses that react near the substrate surface. This reaction results in thin epitaxial material layers developing on the wafer substrate surface, creating a desired crystalline structure thereon. The resulting processed wafers can be used in the production of end products such as, for example, integrated circuits and light emitting diodes.
Compositional uniformity in layers grown on the processed wafers is important to effective wafer growth. For example, some existing substrate processing systems have edge effects at the edge interface between the wafer substrate and the wafer carrier. These edge effects typically are observed after non-uniform heating of the edges of a wafer substrate in the wafer carrier relative to the rest of the wafer substrate. Disadvantageously, edge effects can result in not only non-uniform deposition of epitaxial growth layers on the wafer, but also migration of impurities into parts of the wafer, and even, in some circumstances, warping of the wafer into a “potato-chip” like shape. This can result in reduced wafer yield, reduced wafer quality, and increases in defects and flaws in processed wafers.
This is especially true for deposition processes employing more volatile components such as In, As, P, Mg, Te, Se, S and Zn, among others. These more volatile components may have significant vapor pressure at the temperatures used for growing epitaxial layers on substrate wafers, and may have slightly different gas phase equilibriums over areas of a wafer substrate at slightly different temperatures. Thus, mass transport may occur from one region of the wafer to another due to non-uniform heating of the wafer substrate. Specifically, as a gas flow travels from a hotter region to a slightly cooler region on a wafer, the gas phase in the hotter region may become enriched compared to equilibrium and cause mass transport from one wafer region to another—resulting in a slightly asymmetric increase in composition of the volatile species in the solid phase deposed on the wafer surface in some regions, but not in others. This mass transport from hotter to cooler regions may cause compositional non-uniformity, particularly for substrate materials containing In.